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Lead (Pb) exposure induces testicular damage and infertility. The aim of this study was to analyze and compare the therapeutic effects of antioxidants or vitamin D and calcium, which have previously been shown to reduce the toxic effects of Pb co-exposure, in rats. Rats were exposed to Pb for 28 days and subsequently treated with antioxidant (melatonin, silymarin), vitamin D and calcium (VitDCa) or a combination (melatonin or silymarin with VitDCa) for 28 days. Control groups included untreated rats (no Pb exposure or therapy), rats exposed only to melatonin or silymarin and rats exposed to Pb without post exposure therapy. Pb exposure induced testicular damage, increased blood lead level (BLL) and reduced serum testosterone level (STL). Rats exposed to Pb and left untreated for 28 days showed persistent pathological testicular alterations. The two treatments that were most effective in reversing pathological testis damage and restoring spermatogenesis were melatonin and silymarin. However, silymarin and melatonin treatment resulted in significantly different serum testosterone levels in rats. Whereas melatonin therapy reduced serum testosterone to levels lower than those in control rats, silymarin increased serum testosterone to levels higher than those in controls. Our pathological analysis of testes revealed that melatonin promoted spermatogenesis and regression of Pb exposure-induced degenerative changes, despite the associated reduction in serum testosterone levels. This result suggests that circulating testosterone may not have an important role in spermatogenesis. Collectively, our results suggest that melatonin and silymarin are effective therapies against the toxic effects Pb exposure in the male reproductive system.

We have applied the docking methodology to characterize the binding modes of the divalent metal transporter 1 (DMT1) and the zinc transporter 8 (ZIP8) protein channels with: melatonin, some melatonin metabolites, and a few lead complexes of melatonin and its metabolites, in three different coordination modes (mono-coordinated, bi-coordinated and tri-coordinated). Our results show that bi-coordinated and tri-coordinated lead complexes prefer to bind inside the central region of ZIP8. Moreover, the interaction strength is larger compared with that of the free melatonin and melatonin metabolites. On the other hand, the binding modes with DMT1 of such complexes display lower binding energies, compared with the free melatonin and melatonin metabolites. Our results suggest that ZIP8 plays a major role in the translocation of Pb, bi or tri coordinated, when melatonin metabolites are present. Finally, we have characterized the binding modes responsible for the ZIP8 large affinities, found in bi-coordinated and tri-coordinated lead complexes. Our results show that such interactions are greater, because of an increase of the number of hydrogen bonds, the number and intensity of electrostatic interactions, and the interaction overlay degree in each binding mode. Our results give insight into the importance of the ZIP8 channel on lead transport and a possible elimination mechanism in lead detoxification processes. Graphical abstract .

Melatonin has been proposed as an alternative treatment to the usage of EDTA for lead intoxication. In this computational paper, since previous work has not systematically studied the complexes that may be formed in the existing and proposed treatments, we study 45 possible complexes that we suggest may be formed between Pb and some essential metals with melatonin, melatonin metabolites, and EDTA, analyzing the stability and viability of these through the Gibbs free energy of complexation (ΔΔG), molecular orbitals, and energy decomposition analysis at the DFT level of theory PBE/TZ2P. Our findings show that most complexes present exergonic energies of reaction, and thus spontaneous complex formation. In addition, we show that the AMK and 3OHM melatonin metabolites possess electronic and thermodynamic properties adequate to act as lead trapping molecules due to the lower Pauli repulsion energies involved in the complexes they form and their large negative values of ΔΔG. Therefore, it is shown that both melatonin and some of its metabolites may be employed in a viable treatment for lead intoxication through formation of stable Pb-complexes. Graphical abstract Metal complexes formed with EDTA, melatonin, and its main metabolites.

Human lead (Pb) exposure induces many adverse health effects, including some related to lead accumulation in organs. Although lead bio-distribution in the body has been described, the molecular mechanism underlying distribution and excretion is not well understood. The transport of essential and toxic metals is principally mediated by proteins. How lead affects the expression of metal transporter proteins in the principal metal excretory organs, i.e., the liver and kidney, is unknown. Considering that co-administration of melatonin and lead reduces the toxic effects of lead and lead levels in the blood in vivo, we examined how lead and co-administration of lead and melatonin affect the gene and protein expression of metal transporter proteins (ZIP8, ZIP14, CTR1 and DMT1) in these organs. Rats were exposed intraperitoneally to lead or lead-melatonin. Our results show that Pb exposure induces changes in the protein and gene expression of ZIP8, ZIP14 and CTR1. Alterations in the copper/zinc ratio found in the blood, liver and kidney were likely related to these changes. With DMT1 expression (gene and protein), a positive correlation was found with lead levels in the kidney. Co-administration of melatonin and lead reduced lead-induced DMT1 expression through an unknown mechanism. This effect of melatonin relates to reduced lead levels in the blood and kidney. The metal transport protein function and our results suggest that DMT1 likely contributes to lead accumulation in organs. These data further elucidate the effects of lead on Cu and Zn and the molecular mechanism underlying lead bio-distribution in animals.

Melatonin, a hormone known for its effects on free radical scavenging and antioxidant activity, can reduce lead toxicity in vivo and in vitro.We examined the effects of melatonin on lead bio-distribution. Rats were intraperitoneally injected with lead acetate (10, 15 or 20mg/kg/day) with or without melatonin (10mg/kg/day) daily for 10 days. In rats intoxicated with the highest lead doses, those treated with melatonin had lower lead levels in blood and higher levels in urine and feces than those treated with lead alone, suggesting that melatonin increases lead excretion. To explore the mechanism underlying this effect, we first assessed whether lead/melatonin complexes were formed directly. Electronic density functional (DFT) calculations showed that a lead/melatonin complex is energetically feasible; however, UV spectroscopy and NMR analysis showed no evidence of such complexes. Next, we examined the liver mRNA levels of metallothioneins (MT) 1 and 2. Melatonin cotreatment increased the MT2 mRNA expression in the liver of rats that received the highest doses of lead. The potential effects of MTs on the tissue distribution and excretion of lead are not well understood. This is the first report to suggest that melatonin directly affects lead levels in organisms exposed to subacute lead intoxication., (Copyright © 2015 Elsevier Ireland Ltd. All rights reserved.)

We have applied the docking methodology to characterize the binding modes of the divalent metal transporter 1 (DMT1) and the zinc transporter 8 (ZIP8) protein channels with: melatonin, some melatonin metabolites, and a few lead complexes of melatonin and its metabolites, in three different coordination modes (mono-coordinated, bi-coordinated and tri-coordinated). Our results show that bi-coordinated and tri-coordinated lead complexes prefer to bind inside the central region of ZIP8. Moreover, the interaction strength is larger compared with that of the free melatonin and melatonin metabolites. On the other hand, the binding modes with DMT1 of such complexes display lower binding energies, compared with the free melatonin and melatonin metabolites. Our results suggest that ZIP8 plays a major role in the translocation of Pb, bi or tri coordinated, when melatonin metabolites are present. Finally, we have characterized the binding modes responsible for the ZIP8 large affinities, found in bi-coordinated and tri-coordinated lead complexes. Our results show that such interactions are greater, because of an increase of the number of hydrogen bonds, the number and intensity of electrostatic interactions, and the interaction overlay degree in each binding mode. Our results give insight into the importance of the ZIP8 channel on lead transport and a possible elimination mechanism in lead detoxification processes. [ABSTRACT FROM AUTHOR]